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Sound Detector Hookup Guide

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Introducing The Sound Detector

The Sound Detector

The Sound Detector is a small board that combines a microphone and

some processing circuitry. It provides not only an audio output, but also a

binary indication of the presence of sound, and an analog representation of

it’s amplitude.

Covered in This Tutorial

This tutorial will guide you through hooking up and using the Sound

Detector. It will examine how the circuit works, explain some details about

getting the best performance from the Sound Detector, then present two

different projects that demonstrate how to use it.

Suggested Reading

• How to Use a Breadboard

• Analog vs. Digital

• The Wikipedia article on sound.

Quick Start

To get started with the Sound Detector, simply connect it to a power supply.

(Sound Detector → Power Supply )

• GND → Supply Ground.

•VCC → Power supply voltage between 3.5 and 5.5 Volts. 5 Volts is

ideal.

In a quiet room, power the board up, and then speak into the microphone.

You should see the red LED on the board blink in response to your voice.

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With it’s 3 outputs, the board itself is a lot more flexible. To explore that

flexibility, read on.

Looking Closer

Three Outputs?

The Sound Detector has 3 separate outputs. It’s easiest to see what each is

doing with a graph. The following illustrates how the sound detector

responds to a series of sound pulses.

Sample Sound Detector Output

This shows the output voltages over time.

• The dark green trace is the audio output of the sound detector. The

audio voltage directly from the microphone is found at this output.

• The light green trace is the envelope output. This analog voltage

traces the amplitude of the sound. Of particular interest, notice that

the third pulse gets noticeably louder as it goes.

• Finally, the red line is the gate output. This output is low when

conditions are quiet and goes high when sound is detected.

How It Works

Having examined the outputs, lets also take a quick walk through the

schematic, to gain an understanding of how each stage works.

First Stage

The first section of the circuit is an electret microphone capsule. This

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portion of the circuit borrows from the Electret Microphone breakout board.

Microphone and Preamplifier

The capsule is biased by the supply voltage through R1, and it outputs an

AC voltage that is riding a DC offset of approximately ½ the supply voltage.

The output from the capsule is an extremely small voltage, so the signal

from the capsule is amplified by IC1G1, an operational amplifier stage. By

default, the preamplifier has an arithmetic gain of 100 (20 dB), and the gain

can be adjusted by populating R17 (which we’ll examine in detail on the

next page).

The audio output is DC coupled, riding one half the supply voltage, so it

can be directly connected to the ADC of a microcontroller. In perfectly quiet

conditions, it will ideally read ½ full scale, or 512 on a 10-bit converter.

Second Stage

The second stage of the circuit is an envelope follower.

Envelope Follower

IC1G3 forms an opamp-based precision rectifier. This stage implements the

equation

if(Vin > 0)

Vout = 0;

else

Vout = Vin * 2.2

The opamp inverts and amplifies the signal. When it’s output swings high,

D2 turns on, and charges C1. When the opamp output is high or not

swinging, D2 is turned off, and C1 discharges through R9. Thus, C1 tracks

the peaks of the input signal.

IC1G4 is a buffer amplifier, so external loads on the envelope pin won’t

change the C1’s charge/discharge behavior.

This results in a signal that tracks the peak amplitude of the input signal. A

louder sound will result in a higher voltage on the Envelope pin. As with the

audio pin, the envelope can be connected to the ADC of a microcontroller.

Third Stage

The final stage implements a thresholded switch on the envelope signal.

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Schmitt Trigger

The Schmitt trigger watches the envelope signal, and toggles the output

when the threshold is exceeded. A Schmitt trigger is a comparator that

adjusts it’s threshold voltage when the output switches, requiring a higher

voltage to switch on than to switch off. This allows it to ignore some ripple in

the input signal, like the ripple present in the output of the envelope follower

stage.

The output of the Schmitt trigger is found on the Gate pin. You can connect

it to a digital input. We’ll use it to trigger interrupts in the software example.

Outputs

Each of the three output signals is present on the .1" header at the edge of

the board. They are active simultaneously. If you aren’t using one in your

particular application, simply leave that pin disconnected.

Configuration

Care and Feeding Of The Capsule

The heart of the Sound Detector is the electret microphone capsule –

without it, we couldn’t convert acoustic energy into electrical energy. These

capsules have a couple of quirks that we need to understand in order to

apply them successfully.

Inside the capsule is the diaphragm, which is actually one plate of a small

capacitor. That capacitor forms a voltage divider with the external bias

resistor. The diaphragm moves in response to sound, and the capacitance

changes as the plates get closer together or farther apart, causing the

divider to change. Since capacitors are sensitive to loading, it’s internally

buffered with a JFET (junction field-effect transistor).

Due to the mechanical and electronic tolerances involved, some capsules

are more sensitive than others. Also, the JFET is rather sensitive to noise

on the power supply. Both of these factors need to be accounted for when

deploying the Sound Detector.

Power Supply

The Sound Detector is an analog circuit, and as such, it’s more sensitive to

noise on the power supply than most digital circuits. Since the capsule is

effectively a voltage divider sitting across the power rails, it will transcribe

any noise on the supply lines onto the capsule output. The next circuit in

the chain is a high-gain amplifier, so any noise on the supply will then be

amplified. Therefore, the Sound Detector may require more careful power

supply configuration than many circuits.

In testing with various supplies, a significant degree of variability was

discovered - some supplies are less noisy than others. One exhibited as

much as 30 mV ripple on the supply output, an as a result, the the Sound

Detector was rather sensitive and unstable. You can check how clean a

power supply is by checking it with an oscilloscope or volt meter, set to the

AC Volts (or, if provided, the AC millivolts) range. A truly clean supply will

show 0.000 VAC. Based on the supplies used in testing, ripple of more than

about 10 mV is problematic.

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Powering my Arduino with a 9V external supply, which allows the onboard

regulators to function, the Arduino’s 5V output was sufficiently clean.

However, powering it from the 5V available on the USB port on a PC, the

regulators are bypassed, and the results were somewhat less usable, and

vary greatly between different ports on different PCs. A powered USB hub

will probably provide cleaner power than the ports on the PC itself.

If all else fails, three 1.5V batteries in series make a nice, clean source of

4.5V.

Amplitude Calibration

The Sound Detector comes set for moderate sensitivity - speaking directly

into the microphone, or clapping your hands nearby should cause the gate

output to fire. If you find that it doesn’t work well in a specific application,

you can change the circuit to be more or less sensitive.

The gain is set by changing the feedback resistors in the preamp stage.

The resistors are marked in the silkscreen on the PCB.

Resistor Closeup

R3 is a surface mount part, with 100K Ohm populated by default. R17 is an

unpopulated position for a through hole resistor.

Lowering The gain

It’s most likely that you’ll find the detector to be too sensitive. In testing the

board for this writeup, noisy air conditioning and music in the next office

were enough to set it off. To make the board less sensitive, you can lower

the preamplifier gain by populating R17 in parallel with R3.

R3 Value R17 Value Arithmetic Gain Gain (dB)

100K - 100 40

100K 100K 50 33

100K 47K 32 30

100K 22K 18 25

100K 10K 9 19

100K 4.7K 4 13

100K 2.2K 2 6

Raising The Gain

If you want to make the sound detector more sensitive, so that it will be

activated by quieter sounds, you can remove R3, and populate R17.

R17 Value Arithmetic Gain Gain (dB)

100K 100 40

220K 220 46

470K 470 53

1Meg 1000 60

Lights Out

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Sound Detector Arduino Example SparkFun Wish List

In some applications, the onboard LED may be distracting or undesirable.

To disable it, simply use a solder sucker or wick to remove the solder blob

from the jumper near the LED.

Physical Considerations

The electret capsule on the Sound Detector is also sensitive to mechanical

vibration and wind noise.

The Sound Detector will pick up handling noise. Mounting it with a resilient

material can help absorb vibration – placing it on a piece of open-cell foam

helped reject vibration conducted through the testing workbench. Other

resilient mounting strategies would include suspending it using rubber

bands, or building shock absorbers using #4 machine screws and heat-

shrink tubing.

Wind noise is caused when blasts of air enter the capsule. Examples would

be a windy day, or the sharp exhalation that often accompanies syllables

such “p,” “b” and “t.” The capsule come with a thin fabric covering, but it

may not be enough to prevent stronger blasts. You can craft a more robust

windscreen with open-cell foam or synthetic fur (like the covering that

sneaks into the frame when a boom mic accidentally enters a film scene).

Software Example

Now that we understand how to configure the board, let’s hook it to an

Arduino and see what it can do.

Materials

In addition to the Sound Detector, you’ll need the following parts.

Jumper Wires Premium 12" M/F Pack of 10

PRT-09385

This is a SparkFun exclusive! These are 12" long jumper wires termin…

Break Away Male Headers - Right Angle

PRT-00553

A

row of right angle male headers - break to fit. 40 pins that can be cu…

RedBoard - Programmed with Arduino

DEV-11575

A

t SparkFun we use many Arduinos and we're always looking for the …

Connections

We snapped a 5-pin section off the header, and soldered it to the Sound

Detector. Then we took the jumper wires and made the following

connections.

(Sound Detector → Arduino )

• GND → Supply Ground

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•VCC → Power supply voltage between 3.5 and 5.5 Volts

• Gate → Pin 2

• Envelope → A0

Additionally, as described on the calibration page, a 33K Ohm resistor was

soldered into position R17. R3 was left in place, so the gain was lowered

from 40 dB to about 28 dB.

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/*************************************************************

*****************

*sound_detector_demo.ino

*Sounddetectorsamplesketch

*ByronJacquot@SparkFunElectronics

*February19,2014

*https://github.com/sparkfun/Sound_Detector

*

*ThissketchdemonstratestheuseoftheSparkfunSoundDete

ctorboard.

*

*TheSoundDetectorisasmallboardthatcombinesamicroph

oneandsome

*processingcircuitry.Itprovidesnotonlyanaudiooutpu

t,butalsoa

*binaryindicationofthepresenceofsoundandananalogre

presentation

*ofit'samplitude.

*

*Thissketchdemonstratestwodifferentmodesofusagefort

heSound

*Detector.Thegateoutput(abinaryindicationthatishig

hwhensound

*ispresent,andlowwhenconditionsarequiet)isusedtof

ireapinchange

*ISR,whichlightsanLEDwhenthesoundispresent.Theen

velopeoutput

*(ananalogvoltagetorisestoindicatetheamplitudeofth

esound)is

*sampledintheloop(),anditprintsanindicationofthel

eveltothe

*serialterminal.

*

*FormoredetailsabouttheSoundDetector,pleasecheckth

ehookupguide.

*

*Connections:

*TheSoundDetectorisconnectedtotheAdrduinoasfollows:

*(SoundDetector>Arduinopin)

*GND→GND

*VCC→5V

*Gate→Pin2

*Envelope→A0

*

*Resources:

*Additionallibraryrequirements:none

*

*Developmentenvironmentspecifics:

*UsingArduinoIDe1.0.5

*TestedonRedboard,3.3v/8MHzand5v/16MHzProMinihardwar

e.

*

*Thiscodeisbeerware;ifyouseeme(oranyotherSparkFu

nemployee)atthe

*local,andyou'vefoundourcodehelpful,pleasebuyusar

ound!

*

*Distributedasis;nowarrantyisgiven.

*************************************************************

*****************/

//Definehardwareconnections

#definePIN_GATE_IN2

#defineIRQ_GATE_IN0

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#definePIN_LED_OUT13

#definePIN_ANALOG_INA0

//soundISR()

//Thisfunctionisinstalledasaninterruptserviceroutine

forthepin

//changeinterrupt.Whendigitalinput2changesstate,thi

sroutine

//iscalled.

//Itqueriesthestateofthatpin,andsetstheonboardLED

toreflectthat

//pin'sstate.

void soundISR()

{

int pin_val;

pin_val=digitalRead(PIN_GATE_IN);

digitalWrite(PIN_LED_OUT,pin_val);

}

void setup()

{

Serial.begin(9600);

//ConfigureLEDpinasoutput

pinMode(PIN_LED_OUT,OUTPUT);

//configureinputtointerrupt

pinMode(PIN_GATE_IN,INPUT);

attachInterrupt(IRQ_GATE_IN,soundISR,CHANGE);

//Displaystatus

Serial.println("Initialized");

}

void loop()

{

int value;

//Checktheenvelopeinput

value= analogRead(PIN_ANALOG_IN);

//Convertenvelopevalueintoamessage

Serial.print("Status:");

if(value<= 10)

{

Serial.println("Quiet.");

}

else if((value>10)&& (value<= 30))

{

Serial.println("Moderate.");

}

else if(value> 30)

{

Serial.println("Loud.");

}

//pausefor1second

delay(1000);

}

This code simultaneously demonstrates two different operating modes of

the Sound Detector.

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Sound Detector Analog Example SparkFun Wish List

• First, using the external interrupt facility, the Arduino observes the

gate output, and sets the onboard (pin 13) LED to follow the gate

status.

• Second, in the loop() routine, it uses an analog input to periodically

sample the envelope signal. That value is interpreted into a message

indicating the current loudness via a series of thresholds.

Analog Example

As a purely analog circuit, the Sound Detector isn’t limited to strictly being a

peripheral for a microcontroller. To illustrate an alternate application, we’ve

wired up a completely analog example. We’ve tied the envelope output to

an LM3916 VU bar-graph LED driver, to make a visual sound level meter.

For this application, the Sound Detector is configured with no resistor for

R17, and the default 100K in position R3.

Materials

Again, we start with the Sound Detector, then add the following parts.

Breadboard - Classic

PRT-00112

Your first exposure to electrical engineering - the bread board. Who k…

Wall Adapter Power Supply - 12VDC 600mA

TOL-09442

This is a high quality AC to DC 'wall wart' which produces a regulated …

10 Segment LED Bar Graph - Red

COM-09935

These 10 segment bar graph LEDs have many uses. With a compact …

Resistor Kit - 1/4W (500 total)

COM-10969

Resistors are a good thing, in fact, they're actually crucial in a lot of ci…

Voltage Regulator - Adjustable

COM-00527

This is the LM317TG voltage regulator, a three-terminal positive regul…

(2) Electrolytic Decoupling Capacitors - 10uF/25V

COM-00523

Low-profile electrolytic decoupling capacitors 10uF/25V. These capac…

Dot/Bar Display Driver - LM3916 (VU Taper)

COM-12695

This is the LM3916 Dot/Bar display analog-controlled LED driver that …

Schematic

We connect those parts as follows.

Which translates thusly onto a solderless breadboard.

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The assembled, operational version looks like this.

Listening to “The Lonely Bull” by the Ventures

Sound picked up by the microphone is now translated onto the bar graph.

The louder the sound is, the more LEDs light up!

A few of notes about the circuit.

• It uses two power supply rails. An external supply was used as a

source of 13 VDC. The 13V is further regulated down to 5V using an

LM317 adjustable regulator. This provides the recommended

minimum of 12V for the bar-graph driver, and a stable source of 5V

for the Sound Detector.

◦The 5V is connected to the + rail at the top of the breadboard.

It is used to power the Sound Detector, and as the anode

supply for the LEDs.

◦The 13V is on the lower + rail, and is the power supply for the

LM3916.

• The 100K Ohm resistor between pins 6 and 7 of the LM3916 sets the

reference for the scaling of the bar graph. If the meter seems to be

too sensitive, it can be reduced to 10K or lower.

• Pin 6 of the LED bar graph is at one of the covered-over positions on

the power rail. The leg was folded over to make contact with pin 5.

• Pin 9 of the LM3916 sets the chip for dot or bar modes. As shown

above, tied to the 13V rail, the chip is in bar mode. If pin 9 is left

unconnected, the chip will be in dot mode.

Resources and Going Further

A few parting thoughts.

Troubleshooting:

If the Sound Detector is misbehaving, try the following steps.

• Double check that the power supply is clean, preferably with an

oscilloscope. Irregularities on the supply will likely be present on the

output.

• The gain configuration is easier to tame with the higher headroom

offered by higher supply voltages. Providing the Sound Detector with

a stable 5V supply, and translating output down to 3.3V will yield

better results than powering the detector directly from 3.3V.

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• Finally, listen to the audio output. AC-couple the audio output using a

10 uF electrolytic capacitor, with its + leg to the sound detector, and

connect a small speaker or headphone. Simply listening to the output

often yields clues as to what’s wrong.

◦You’ll be able to hear whether unexpected noises, such as

wind and vibration, are being picked up.

◦If the audio output is distorted, the gain may need to be

reduced.

◦If the audio output is really quiet, the gain may need to be

increased.

◦If there is a constant tone on the audio output, double-check

that the power supply is suitably stable. Switch-mode power

supplies often introduce oscillations on the power rails.

Documentation

Design files for the Sound Detector are in the corresponding GitHub

repository. This includes the Eagle files, and the Arduino example and

Fritzing example we explored above.

Additionally, there are SPICE simulations of the circuit for LT Spice. One of

those simulations, sound_detectorwav.asc actually analyzes the contents

of a wav file (which was used to generate the diagram on page 2 of this

tutorial), though you’ll have to modify the file path to analyze a wav file if

your own.

Finally, the Sound Detector has been added to the sensors category in the

Sparkfun Fritzing Library.

See Also

• The Wikipedia page for electret microphones

• LM3914/5/6 Dot/Bar Display Driver Hookup Guide

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